The thin natural oxide film which forms on tin surfaces provides a useful protective barrier and improves paint adhesion. If uncontrolled, however, the thin oxide film will form a thick yellow non-protective and non-adhesive layer. Therefore, a goal of tin producers has been to provide tin with an oxide film while preventing its rapid uncontrolled growth to a thick layer which prevents the effective adhesion of paint to the tin surface. Tin producers have passified tin surfaces in various ways to provide tin with a thin oxide film while preventing its rapid uncontrolled growth. In addition, proper passification of tin surfaces prevents or substantially reduces sulfide stains.
Previous attempts at passification have mainly centered around thickening the natural oxide film with an oxidant while leaving a corrosion resistant film on the surface of the metal to retard further oxide growth and prevent sulfide stains. U.S. Pat. No. 1,827,204, discloses an electrolytic process which uses chromates to both thicken the oxide film and leave a film of reduced chromic oxides to prevent further oxide growth and prevent sulfide stains. French Patent 777,314, Tichauer, discloses a process which used sodium molybdates as an oxidant and various heavy metals to provide a passive film on the surface of the metal. U.S. Pat. No. 2,024,951 discloses a process which uses potassium permanganate to both stabilize the oxide film and reduce sulfide staining. In 1940, U.S. Pat. No. 2,215,165, disclosed an electrolytic process which oxidized and then reduced the tin surface to thicken the oxide film and leave a passive tin surface. That same year Ken R., Protective Films on Tin Plate by Chemical Treatment, J. Sol. Chem. Ind., 59, 259 C 1940, disclosed the results of an investigation which showed that an alkaline solution of chromates would passivate tin surfaces. In 1943, U.S. Pat. No. 2,312,076, disclosed a process which used dichromates mixed with phosphates to passivate tin surfaces. Since that time new patents, i.e., U.S. Pat. No. 2,606,866, have been issued, but have all centered around improvements in the basic dichromate/chromic acid processes mentioned.
A major drawback of the prior arts chromium containing coating compositions is their reliance on hexavalent chromium. Hexavalent chromium is extremely toxic and thus more costly to work with. For instance, hexavalent chromium requires special waste disposal.